1. Field of the Invention
The invention relates to a powdering unit having a nozzle strip which produces a powder gas curtain containing powder particles distributed in a carrier gas stream, which curtain moves substantially in a curtain plane. It relates furthermore to a powdering station comprising two such powdering units. Furthermore, the invention relates to a method for operating a powdering unit and a powdering station.
2. Background Art
Powdering units are used in the printing industry to spray the ink films of printed products which have not yet fully dried with a very fine powder (organic or inorganic) and thus prevent the printed products from sticking together via the ink layers when they are laid on top of one another to form a stack. In the event of blocking between successive printed products via the ink layers, the ink layers would be damaged on separating the individual printed sheets again, rendering the printed products unusable.
In the known powdering units, a powder gas produced in a separate generator and containing powder particles suspended in a carrier gas stream is delivered via a nozzle strip having a multiplicity of nozzles of substantially the same orientation spaced apart in the longitudinal direction of the strip. The nozzle strip thus produces a powder curtain which is substantially homogeneous at a certain distance from the nozzle strip and through which the printed products to be sprayed with powder are moved with a conveying direction substantially perpendicular to the mean direction of the powder curtain.
The powder curtain diverges substantially in a wedge shape from the nozzle strip. The mid-plane of this powder gas curtain is referred to hereinbelow and in the claims as curtain plane for short.
In such powdering units, it is sought first of all to guide the powder curtain in such a way that as many as possible of the powder particles entrained in the carrier gas stream come into intimate contact with the ink layer of the printed product and adhere to the ink layer when and where the latter is still tacky.
By means of the present invention, a powdering unit having a nozzle strip which produces a powder gas curtain containing powder particles distributed in a carrier gas stream, which curtain moves substantially in a curtain plane, is to be developed in such a way that an even better adherence of the powder particles to the surface of the products to be sprayed with powder is ensured.